The present invention relates to glass ceramic precursor compositions, and more particularly to glass ceramic precursor compositions containing titanium diboride which upon firing form glass ceramic materials that are useful, for example, as high temperature resistant protective coatings on a substrate.
Many materials which have usable strengths at elevated temperatures and under adverse conditions decompose when exposed to harsh environments. For example, graphites will oxidize to carbon dioxide and ferrous metals will scale. Moreover, most materials will react with their environments at higher temperatures. Efforts have been made to prevent adhesion or reactions at higher temperatures by altering the surface quality of degradable materials which otherwise have desirable properties. For example, high temperature resistant protective coatings are known which serve to extend the life of various materials, including graphite. These coatings are fused to the surface of the host material at high temperature and controlled pressure and, as such, are costly and require expensive coating and fabrication equipment.
Protective coatings which are not fused have been developed which function to some extent at relatively moderate temperatures. These coatings are intended to insulate or encapsulate the surface of the substrate or host material, such as graphite and construction materials, in such a way as to prevent the surface from reacting adversely in a harsh environment, and thereby extending the usable life of such materials particularly at elevated temperatures. Such coatings, as for example, silicon carbide paint specifically designed as a graphite electrode coating, are extremely brittle even at relatively low temperatures and are otherwise unsatisfactory. Another protective coating which has been employed in an effort to extend the life of electrodes is disclosed in U.S. Pat. No. 4,487,804 and uses a silicaphosphorous glass composition modified with titanium dioxide and no boron component. Relatively low percentages of titanium diboride have been used to form protective coatings for steel, but only in combination with chromium oxide and titanium dioxide.
Titanium diboride has also been used in the past with graphite electrodes, but not as a graphite protective coating. Joo' et al., in their U.S. Pat. No. 4,439,382, describe a process for manufacturing a graphite-titanium diboride composite useful as a cathodic element to provide improved electrical efficiency over prior amorphous carbon and semi-graphite cathodes. While corrosion of a cathode element is improved, the life of the cathode is not significantly extended.
Other relevant prior art coating compositions include a variety of high temperature resistant coatings primarily based upon aqueous solutions of metal silicates, such as water glass, aqueous solutions of phosphates including phosphoric acid and metal salts of the acid, and various tar and graphite mixtures to form an additional layer of carbon on graphite substrates. Some coatings have been applied in a sintering or pressing fashion forming a hermetic barrier layer. Solvent systems have several disadvantages due in substantial measure to the porosity which inevitably results as the solvent leaves the system and permits air to diffuse to the substrate. As an added drawback, the sintered type coatings require extensive equipment and design considerations as they substantially alter the finished product and manufacturing techniques.
It is, accordingly, an object of this invention to provide a coating that will insulate or encapsulate the surface of a substrate or host material to form a protective barrier in harsh environments.
It is also an object of this invention is to provide a coating that will lengthen the usable life of various materials at elevated temperatures.
It is another object of this invention to provide a hermetic sealant or binder, particularly on that will seal glass to metal components.
It is yet another object of this invention to provide a material that is used to manufacture parts of articles.